1. Field of the invention
The present invention relates to a fluid pressure control apparatus for wheel brakes, more specifically, to a fluid pressure control apparatus which generates a braking force by supplying a fluid pressure generated by a pressure source to a wheel brake based on information representative of a driving state of a vehicle detected by a wheel speed sensor or a distance sensor. The fluid pressure control apparatus of the present invention is especially used to keep the vehicle away from a preceding vehicle by a certain distance.
The construction of a conventional fluid pressure control apparatus varies in many respects. According to a conventional foot-operated braking apparatus, a fluid pressure generated in a master cylinder is supplied to a wheel brake upon depression of a brake pedal. However, the fluid pressure control apparatus disclosed, for example, in Japanese Patent Application Laid-Open No. HEI 6-199215 or Japanese Utility Model Laid-Open No. HEI 7-35221 automatically supplies a fluid pressure generated by a pressure source other than the master cylinder to the wheel brake, regardless of whether the brake pedal is depressed or not.
In the above-mentioned fluid pressure control apparatus, an automatic braking actuator is installed such that a brake fluid supplied from the master cylinder and that supplied from the pressure source flow through the same passage. The pressure source includes a pump driven by a drive motor and an accumulator for accumulating a fluid-pressure generated by the pump. When the automatic braking process is triggered, the fluid pressure accumulated in the accumulator is supplied to the automatic braking actuator.
Since this fluid pressure control apparatus necessitates a relatively bulky accumulator as a pressure source, it is disadvantageous in terms of installation.
Another fluid pressure control circuit disclosed in Japanese Published Patent No. HEI 5-65388 employs a pump as a pressure source and ensures a sufficient fluid pressure without using the accumulator. However, this fluid pressure control circuit is only capable of generating a fluid pressure required for traction control (TRC). This fluid pressure control circuit allows the entire amount of fluid discharged by the pump to be supplied to the wheel brake, and thus is unsuitable for control processes in which a gradient of pressure increase has to be finely adjusted in accordance with the circumstances surrounding the vehicle.
For this purpose, a variable capacity pump may be used. However, the use of the variable capacity pump is equally unsuitable because it has a substantially complicated structure, is very expensive, and requires a space almost as large as that for accommodating an accumulator.
2. Description of the Related Art